Light-emitting diode (LED) based illumination devices are increasingly used for a wide variety of lighting applications. LEDs offer advantages over traditional light sources, such as incandescent and fluorescent lamps, including long lifetime, high lumen efficacy, low operating voltage and fast modulation of lumen output.
Efficient high-power LEDs are often based on blue light emitting materials. To produce an LED based illumination device having a desired color (e.g., white) output, a suitable wavelength converting material, commonly known as a phosphor, may be used which converts part of the light emitted by the LED into light of longer wavelengths so as to produce a combination of light having desired spectral characteristics. The wavelength converting material may be applied directly on the LED die, or it may be arranged at a certain distance from the phosphor (so-called remote configuration).
Many inorganic materials have been used as phosphor materials for wavelength conversion. However, many inorganic phosphors, in particular red phosphors, suffer from the disadvantage that the conversion spectrum peak is relatively wide, and that a large amount of infrared or near-infrared light is generated, to which the human eye has low or no sensitivity. Another important disadvantage of many conventional phosphors is the relatively large Stokes' loss (the energy difference between the absorbed wavelength and the emitted wavelengths). The Stokes' loss, but also the emission of infra-red wavelengths, may cause problems with regard to thermal management, since heat absorbed by the phosphor must be carried off in order to avoid degeneration and thermal quenching of the wavelength converting material.
In order to avoid generation of large amount of infra-red radiation, the use of red quantum dots has been proposed. Quantum dots have a narrow conversion (emission) peak and do not produce infrared light, which increases the efficiency of an LED based device. However, quantum dots, which are usually provided in a transparent matrix of silicone or acrylic polymer, are highly sensitive to oxidation, and must therefore be protected from oxygen and water in order to avoid degradation.
US 2012/0113672 discloses quantum dot encapsulation and matrix materials, including quantum dot films with protective barriers. However, a general problem with various known encapsulations is that the barrier material either has undesirably low light transmission, which reduces the device efficiency, or that the barrier materials are highly expensive, which effectively prevents their use in many applications.